For the purpose of sexual reproduction, eukaryotic organisms produce specialised cells called gametes. These cells typically contain half the number of chromosomes as somatic and precursor germ cells and so are said to be haploid. They are derived from diploid germ cells via the two successive divisions that constitute meiosis. This series of events not only halves the chromosome number but also, through the processes of independent assortment and crossing-over, ensures that no two gametes contain the same complement of alleles. Meiosis is therefore a source of enormous genetic diversity, the raw material for natural selection.
Interphase
The period in the cycle of a germ cell before it undergoes meiosis is known as interphase. During this period, as in mitosis, (nuclear division in which chromosome number is preserved), the cell grows and each pair of homologous chromosomes is replicated, producing pairs of identical sister chromatids bound along their lengths by cohesin complexes. The cell now contains four copies of each chromosome. Figure 1 shows a nucleus in interphase, with diffuse chromosomes not yet individually visible and the nucleolus, (dark spot at the lower left of the nucleus), and nuclear envelope still intact.
Figure 1: Light micrograph of a locust germ cell at interphase.
Prophase I
The first phase of the first meiotic division is analogous to, but more complicated than, prophase in mitosis and so is further divided into five parts: leptotene, zygotene, pachytene, diplotene and diakinesis.
Leptotene
From the Greek meaning “thin thread”, in this stage, as can be seen in figure 2, the pairs of sister chromatids are beginning to condense, becoming visible under the light microscope, though not yet individually identifiable. The nucleolus has broken down at this stage and so is no longer visible.
Figure 2: Light micrograph of a locust germ cell during leptotene of prophase I of meiosis.
Zygotene
Meaning “paired threads”, zygotene is the stage at which the process of synapsis begins, in which homologous pairs of sister chromatids line up together and a protein structure known as the synaptonemal complex begins to form between them, somewhat in the manner of a zipper. This complex consists of several subunits: lateral elements bound to the DNA of each chromatid-pair are joined to transverse filaments, which are themselves linked by a central element. This structure is repeated along the length of the chromatid-pair, which, upon completion of synapsis, is known as a bivalent or tetrad.
Figure 3: Light micrograph of a locust germ cell during zygotene of prophase I of meiosis.
Pachytene
Meaning "thick thread", in pachytene synapsis is complete. Each pair of chromosomes is aligned with one another, forming bivalents. The process of crossing-over now occurs: genetic material is exchanged between each homologous pair, catalysed by the presence of recombination nodules which have formed along the synaptonemal complex. The points at which crossing-over has occurred are known as chiasmata after their resemblance to the Greek letter χ (chi).
Figure 4: Light micrograph of a locust germ cell during pachytene of prophase I of meiosis.
Diplotene
From the Greek for "two threads", in diplotene, each bivalent now has one or more chiasmata, held together by sister chromatid cohesion complex, clearly visible in figure 5. Sister chromatid cohesion is dependent on the protein complex called cohesin, which is interspersed along the length of each sister chromatid, this prevents the pairs moving apart before successful segregation has occurred. The synaptonemal complex dissolves in preparation for anaphase.
Figure 5: Light micrograph of a locust germ cell during diplotene of prophase I of meiosis.
Diakinesis
The tetrads now condense, becoming more visible with clearly distinguishable chiasmata. The membranes of the nuclear envelope disintegrate into small vesicles and the meiotic spindle begins to form.
Figure 6: Light micrograph of a locust germ cell during diakinesis of prophase I of meiosis.
Metaphase I
Although metaphase of meiosis I is similar to metaphase in mitosis and meiosis II, there are crucial differences. Firstly, it is bivalents, rather than homologous pairs of chromosomes that are arranged along the metaphase plate. One chromatid-pair of each bivalent is attached to kinetochore microtubules extending from one pole and the other pair is attached to microtubules on the opposite pole of the spindle, so that they thus segregate together into opposite daughter cells at anaphase I. Secondly, the arrangement of chromatid pairs is random, such that maternal and paternal pairs are independently assorted into daughter cells – a source of great genetic diversity. At this point of meiosis I, the cohesion complex that has been keeping the sister chromatids together is still in place. There is a strong linkage between the homologues that resists the force generated by the depolymerisation of the microtubules at the kinetochore until the bivalents are aligned at the equator of the spindle. The chiasmata that have formed between the maternal and paternal homologues and the cohesion complex that is between the sister chromatid arms, cooperate in holding the homologues together. This holding capacity is vitally important in ensuring that the homologues segregate correctly. The arms of the sister chromatids only separate at anaphase I, which thus resolves the chiasmata and allows the homologues to separate, but the sister chromatids will stay attached at the centromere until meiosis II.
Figure 7: Light micrograph of a locust germ cell during metaphase I of meiosis, (polar view).
Figure 8: Light micrograph of a locust germ cell during metaphase I of meiosis, (side view).
Anaphase I
Anaphase begins with the breakdown of the cohesion complex, which is responsible for sister chromatid cohesion along the chromatids and allows the homologues to separate except in the centromere region. This breakdown, along with the disintegration of the chiasmata, allows the kinetochore microtubules to pull the homologues (the non sister chromatids) toward opposite poles as can be seen in figure 9. This is achieved without input of energy from ATP; the depolymerisation of kinetochore microtubules provides the motive force. As the sister chromatid cohesion complex remains at the centromere, the sister chromatid pairs (maternal or paternal) move as a unit towards the same pole. This is accomplished with proteins called shugoshins. These are proteins that are associated with the kinetochores and ensure that the sister chromatid kinetochore does not dissociate during anaphase I while the proteolytic enzyme separase cleaves the cohesion complex. Shugoshin achieves this by recruiting a phosphatase to the centromere region, which reverses the phosphorylation of the cohesion complexes, which is needed for separase to cleave them; separase is thus inhibited from cleaving the centromere at this stage.
Figure 9: Light micrograph of a locust germ cell during anaphase I of meiosis.
Telophase I
By the beginning of Telophase I, the nuclear envelope has started to reform around the two sets of chromosomes. The chromosome number in these new nuclei is now haploid. Cytokinesis usually occurs simultaneously with telophase I creating two daughter cells. In some species, the chromosomes will now decondense while the nuclear envelope is reforming. An important aspect of the meiotic process is that no chromosome replication occurs between meiosis I and II.
Figure 10: Light micrograph of a locust germ cell during telophase I of meiosis.
Prophase II
Comparable with prophase of mitosis, during prophase II the chromosomes in the two resultant daughter cells either remain condensed or recondense as the meiotic spindle begins to form. Figure 11: Light micrograph of a locust germ daughter cell during prophase II of meiosis.
Metaphase II
Metaphase II follows the same principles as metaphase in mitosis where the sister chromatids align on the spindle equator, (rather than homologous chromosomes as in metaphase I of meiosis). Only now that one cell division has taken place, these stages occur within the two nuclei concurrently. The alignment of the chromatids along the metaphase plate is again completely random. An important aspect of metaphase II is that because of the crossing over that occurred in prophase of meiosis I, the two sister chromatids of each chromosome are not genetically identical. At this point, as in mitosis, the kinetochores of the sister chromatids are attached to microtubules extending from opposite poles of the spindle.
Figure 12: Light micrograph of a locust germ daughter cell during metaphase II of meiosis.
Anaphase II
The beginning of anaphase II is marked by the dissolution of the residual cohesion complex at the centromeres as now the shugoshin protein is not present. This allows the sister chromatids to finally separate. These chromatids move towards opposite poles as individuals to form four haploid daughter cells. Metaphase II and anaphase II usually occur very quickly compared to their analogues in meiosis I.
Figure 13: Light micrograph of a locust germ daughter cell during anaphase II of meiosis.
Telophase II
The nuclei reform and the chromosomes begin to decondense while cytokinesis occurs simultaneously as in Telophase I. There are now four daughter cells, each with a haploid number of chromosomes. Due to crossing over and recombination, each of these cells is genetically distinct from each other.
Figure 14: Light micrograph of a locust germ daughter cell during telophase II of meiosis.
Spermatids
Taking place in the testes, meiosis in the germ line of male animals occurs as a part of spermatogenesis, the production of sperm cells. Meiosis of one spermatocyte, (diploid premeiotic male germ cell, itself derived from spermatogonial stem cells by mitosis), results in four haploid gametes known as spermatids, following telophase of meiosis II. Spermatids are roughly spherical and contain a nucleus, golgi apparatus, centrioles and mitochondria.
Figure 15: Light micrograph of locust spermatids.
Sperm cells
The process by which spermatids mature into motile sperm cells is called spermiogenesis. The process is divided in four phases. In the Golgi phase, the radial spermatid begins to develop polarity. The acrosomal cap develops in the cap phase. In the acrosomal phase, the centrioles elongate to become the tail, and the sperm align themselves so that the tail points away from the epithelium. After the maturation phase, spermiation takes place, which removes unnecessary cytoplasm. The sperms, which are mature but not motile, are transported to the epididymus, where they gain motility in form of a flagellar tail. The morphology of the sperm cells shown in figure 16 is atypical of many animal groups; nuclei are usually much more compact, with most of the length of the cell taken up by the flagellum.
Figure 16: Light micrograph of locust sperm cells.
1. In rhetoric, meiosis is a euphemistic figure of speech that intentionally understates something or implies that it is lesser in significance or size than it really is. The two meanings are not to be confused.
Please use your first year heredity and Gene action notes, the reason I want you to do this task is because unless you understand meiosis, you will not really understand the contribution of meiosis to evolutionary processes.
B. What is the role of polyploidy in evolution, 300 words maximum, referenced.
What is the role of polyploidy in evolution?
Polyploidy is a condition in which cells contain more than two homologous chromosomes (i.e. more than two sets). It is common in plants and widespread in other organisms, species presenting the condition often being successful within their milieu (Comai, 2005). In fact most organisms on Earth are affected in some way by polyploidy. Plants, which make up around 90% of Earth's biomass are mostly polyploid, humankind is fuelled by polyploid cereal endosperm (Bennett, 2004) and two rounds of polyploidy occurred in early vertebrate evolution (Ohno, 1970).
Polyploidy seems to be woven into the evolutionary history of many organisms. This may be due to the raw material for evolution a polyploid event provides. Allopolyploidy provides a mechanism whereby infertile hybrids can, by a doubling of genome, become fertile. This occurs because the resultant chromosomes can pair during meiosis. Not only does this result in instant speciation and the creation of vigorous hybrids but collects all of the allelic variation from both species together providing a huge genepool for phenotypes on which selection can act.
Neofuntionalisation is the appearance of a new function caused by the duplication of genes, trangressive characters are novel phenotypes not posessed by either of the hybrids parents (Schwarzbach et al 2001). These two effects of polyploidy again provide novel and complex phenotypes on which selection can work. It may be factors like these and allopolyploidy that make polyploid plants excellent pioneer species in previously glaciated areas of the Arctic, for example. In this way polyploidy may also have an impact upon successional processes. As more species have their genomes analysed it is likely that many more, previously thought to be diploid, will be found to possess polyploid histories (Lukens et al, 2004).
References:
Bennett, M.D., 2004. Perspectives on polyploidy in plants – ancient and neo. Biological Journal of the Linnean Society. 82, pp. 411-423. Brochmann, C., Brysting, A.K., Alsos, I.G., Borgen, L.,Grundt, H. H., Scheen, A.C. and Elven, R., 2004. Polyploidy in Artic plants. Biological Journal of the Linnean Society. 82, pp. 521-536. Comai, L., 2005. The advantages and disadvantages of being polyploid. Nature Review. 6, pp. 836-846. Lukens, L.N., Quijada, P.A., Udall, J., Pires, C., Schranz, M.E., Osborn, T.C., 2004. Genome redundancy and plasticity within ancient and recent Brassica crop species. Biological Journal of the Linnean Society. 82, pp. 665-674. Ohno, S., 1970. Evolution by Gene Duplication. Berlin: Springer-Verlag. Schwarzbach, E., Donovan, L., Rieseberg, L., 2001. Transgressive character expression in a hybrid sunflower species. American Journal of Botany. 88(2), pp. 270-277.
Group secretary plus email: Alastair Haigh ef08123@qmul.ac.uk
Group members plus email: Chris Clarkson ef08024@qmul.ac.uk Faye Willman ef08297@qmul.ac.uk Carla Jackson ef08085@qmul.ac.uk Umberto bt09373@qmul.ac.uk
Address A and B below
Meiosis[1]
For the purpose of sexual reproduction, eukaryotic organisms produce specialised cells called gametes. These cells typically contain half the number of chromosomes as somatic and precursor germ cells and so are said to be haploid. They are derived from diploid germ cells via the two successive divisions that constitute meiosis. This series of events not only halves the chromosome number but also, through the processes of independent assortment and crossing-over, ensures that no two gametes contain the same complement of alleles. Meiosis is therefore a source of enormous genetic diversity, the raw material for natural selection.
Interphase
The period in the cycle of a germ cell before it undergoes meiosis is known as interphase. During this period, as in mitosis, (nuclear division in which chromosome number is preserved), the cell grows and each pair of homologous chromosomes is replicated, producing pairs of identical sister chromatids bound along their lengths by cohesin complexes. The cell now contains four copies of each chromosome. Figure 1 shows a nucleus in interphase, with diffuse chromosomes not yet individually visible and the nucleolus, (dark spot at the lower left of the nucleus), and nuclear envelope still intact.
Figure 1: Light micrograph of a locust germ cell at interphase.
Prophase I
The first phase of the first meiotic division is analogous to, but more complicated than, prophase in mitosis and so is further divided into five parts: leptotene, zygotene, pachytene, diplotene and diakinesis.
Leptotene
From the Greek meaning “thin thread”, in this stage, as can be seen in figure 2, the pairs of sister chromatids are beginning to condense, becoming visible under the light microscope, though not yet individually identifiable. The nucleolus has broken down at this stage and so is no longer visible.
Figure 2: Light micrograph of a locust germ cell during leptotene of prophase I of meiosis.
Zygotene
Meaning “paired threads”, zygotene is the stage at which the process of synapsis begins, in which homologous pairs of sister chromatids line up together and a protein structure known as the synaptonemal complex begins to form between them, somewhat in the manner of a zipper. This complex consists of several subunits: lateral elements bound to the DNA of each chromatid-pair are joined to transverse filaments, which are themselves linked by a central element. This structure is repeated along the length of the chromatid-pair, which, upon completion of synapsis, is known as a bivalent or tetrad.
Figure 3: Light micrograph of a locust germ cell during zygotene of prophase I of meiosis.
Pachytene
Meaning "thick thread", in pachytene synapsis is complete. Each pair of chromosomes is aligned with one another, forming bivalents. The process of crossing-over now occurs: genetic material is exchanged between each homologous pair, catalysed by the presence of recombination nodules which have formed along the synaptonemal complex. The points at which crossing-over has occurred are known as chiasmata after their resemblance to the Greek letter χ (chi).
Figure 4: Light micrograph of a locust germ cell during pachytene of prophase I of meiosis.Diplotene
From the Greek for "two threads", in diplotene, each bivalent now has one or more chiasmata, held together by sister chromatid cohesion complex, clearly visible in figure 5. Sister chromatid cohesion is dependent on the protein complex called cohesin, which is interspersed along the length of each sister chromatid, this prevents the pairs moving apart before successful segregation has occurred. The synaptonemal complex dissolves in preparation for anaphase.
Figure 5: Light micrograph of a locust germ cell during diplotene of prophase I of meiosis.
Diakinesis
The tetrads now condense, becoming more visible with clearly distinguishable chiasmata. The membranes of the nuclear envelope disintegrate into small vesicles and the meiotic spindle begins to form.
Figure 6: Light micrograph of a locust germ cell during diakinesis of prophase I of meiosis.
Metaphase I
Although metaphase of meiosis I is similar to metaphase in mitosis and meiosis II, there are crucial differences. Firstly, it is bivalents, rather than homologous pairs of chromosomes that are arranged along the metaphase plate. One chromatid-pair of each bivalent is attached to kinetochore microtubules extending from one pole and the other pair is attached to microtubules on the opposite pole of the spindle, so that they thus segregate together into opposite daughter cells at anaphase I. Secondly, the arrangement of chromatid pairs is random, such that maternal and paternal pairs are independently assorted into daughter cells – a source of great genetic diversity. At this point of meiosis I, the cohesion complex that has been keeping the sister chromatids together is still in place. There is a strong linkage between the homologues that resists the force generated by the depolymerisation of the microtubules at the kinetochore until the bivalents are aligned at the equator of the spindle. The chiasmata that have formed between the maternal and paternal homologues and the cohesion complex that is between the sister chromatid arms, cooperate in holding the homologues together. This holding capacity is vitally important in ensuring that the homologues segregate correctly. The arms of the sister chromatids only separate at anaphase I, which thus resolves the chiasmata and allows the homologues to separate, but the sister chromatids will stay attached at the centromere until meiosis II.
Figure 7: Light micrograph of a locust germ cell during metaphase I of meiosis, (polar view).
Figure 8: Light micrograph of a locust germ cell during metaphase I of meiosis, (side view).
Anaphase I
Anaphase begins with the breakdown of the cohesion complex, which is responsible for sister chromatid cohesion along the chromatids and allows the homologues to separate except in the centromere region. This breakdown, along with the disintegration of the chiasmata, allows the kinetochore microtubules to pull the homologues (the non sister chromatids) toward opposite poles as can be seen in figure 9. This is achieved without input of energy from ATP; the depolymerisation of kinetochore microtubules provides the motive force. As the sister chromatid cohesion complex remains at the centromere, the sister chromatid pairs (maternal or paternal) move as a unit towards the same pole. This is accomplished with proteins called shugoshins. These are proteins that are associated with the kinetochores and ensure that the sister chromatid kinetochore does not dissociate during anaphase I while the proteolytic enzyme separase cleaves the cohesion complex. Shugoshin achieves this by recruiting a phosphatase to the centromere region, which reverses the phosphorylation of the cohesion complexes, which is needed for separase to cleave them; separase is thus inhibited from cleaving the centromere at this stage.
Figure 9: Light micrograph of a locust germ cell during anaphase I of meiosis.
Telophase I
By the beginning of Telophase I, the nuclear envelope has started to reform around the two sets of chromosomes. The chromosome number in these new nuclei is now haploid. Cytokinesis usually occurs simultaneously with telophase I creating two daughter cells. In some species, the chromosomes will now decondense while the nuclear envelope is reforming. An important aspect of the meiotic process is that no chromosome replication occurs between meiosis I and II.
Figure 10: Light micrograph of a locust germ cell during telophase I of meiosis.
Prophase II
Comparable with prophase of mitosis, during prophase II the chromosomes in the two resultant daughter cells either remain condensed or recondense as the meiotic spindle begins to form.
Figure 11: Light micrograph of a locust germ daughter cell during prophase II of meiosis.
Metaphase II
Metaphase II follows the same principles as metaphase in mitosis where the sister chromatids align on the spindle equator, (rather than homologous chromosomes as in metaphase I of meiosis). Only now that one cell division has taken place, these stages occur within the two nuclei concurrently. The alignment of the chromatids along the metaphase plate is again completely random. An important aspect of metaphase II is that because of the crossing over that occurred in prophase of meiosis I, the two sister chromatids of each chromosome are not genetically identical. At this point, as in mitosis, the kinetochores of the sister chromatids are attached to microtubules extending from opposite poles of the spindle.
Figure 12: Light micrograph of a locust germ daughter cell during metaphase II of meiosis.
Anaphase II
The beginning of anaphase II is marked by the dissolution of the residual cohesion complex at the centromeres as now the shugoshin protein is not present. This allows the sister chromatids to finally separate. These chromatids move towards opposite poles as individuals to form four haploid daughter cells. Metaphase II and anaphase II usually occur very quickly compared to their analogues in meiosis I.
Figure 13: Light micrograph of a locust germ daughter cell during anaphase II of meiosis.
Telophase II
The nuclei reform and the chromosomes begin to decondense while cytokinesis occurs simultaneously as in Telophase I. There are now four daughter cells, each with a haploid number of chromosomes. Due to crossing over and recombination, each of these cells is genetically distinct from each other.
Figure 14: Light micrograph of a locust germ daughter cell during telophase II of meiosis.
Spermatids
Taking place in the testes, meiosis in the germ line of male animals occurs as a part of spermatogenesis, the production of sperm cells. Meiosis of one spermatocyte, (diploid premeiotic male germ cell, itself derived from spermatogonial stem cells by mitosis), results in four haploid gametes known as spermatids, following telophase of meiosis II. Spermatids are roughly spherical and contain a nucleus, golgi apparatus, centrioles and mitochondria.
Figure 15: Light micrograph of locust spermatids.
Sperm cells
The process by which spermatids mature into motile sperm cells is called spermiogenesis. The process is divided in four phases. In the Golgi phase, the radial spermatid begins to develop polarity. The acrosomal cap develops in the cap phase. In the acrosomal phase, the centrioles elongate to become the tail, and the sperm align themselves so that the tail points away from the epithelium. After the maturation phase, spermiation takes place, which removes unnecessary cytoplasm. The sperms, which are mature but not motile, are transported to the epididymus, where they gain motility in form of a flagellar tail. The morphology of the sperm cells shown in figure 16 is atypical of many animal groups; nuclei are usually much more compact, with most of the length of the cell taken up by the flagellum.
Figure 16: Light micrograph of locust sperm cells.
1. In rhetoric, meiosis is a euphemistic figure of speech that intentionally understates something or implies that it is lesser in significance or size than it really is. The two meanings are not to be confused.
B. What is the role of polyploidy in evolution, 300 words maximum, referenced.
What is the role of polyploidy in evolution?
Polyploidy is a condition in which cells contain more than two homologous chromosomes (i.e. more than two sets). It is common in plants and widespread in other organisms, species presenting the condition often being successful within their milieu (Comai, 2005). In fact most organisms on Earth are affected in some way by polyploidy. Plants, which make up around 90% of Earth's biomass are mostly polyploid, humankind is fuelled by polyploid cereal endosperm (Bennett, 2004) and two rounds of polyploidy occurred in early vertebrate evolution (Ohno, 1970).
Polyploidy seems to be woven into the evolutionary history of many organisms. This may be due to the raw material for evolution a polyploid event provides. Allopolyploidy provides a mechanism whereby infertile hybrids can, by a doubling of genome, become fertile. This occurs because the resultant chromosomes can pair during meiosis. Not only does this result in instant speciation and the creation of vigorous hybrids but collects all of the allelic variation from both species together providing a huge genepool for phenotypes on which selection can act.
Neofuntionalisation is the appearance of a new function caused by the duplication of genes, trangressive characters are novel phenotypes not posessed by either of the hybrids parents (Schwarzbach et al 2001). These two effects of polyploidy again provide novel and complex phenotypes on which selection can work. It may be factors like these and allopolyploidy that make polyploid plants excellent pioneer species in previously glaciated areas of the Arctic, for example. In this way polyploidy may also have an impact upon successional processes. As more species have their genomes analysed it is likely that many more, previously thought to be diploid, will be found to possess polyploid histories (Lukens et al, 2004).
References:
Bennett, M.D., 2004. Perspectives on polyploidy in plants – ancient and neo. Biological Journal of the Linnean Society. 82, pp. 411-423.
Brochmann, C., Brysting, A.K., Alsos, I.G., Borgen, L.,Grundt, H. H., Scheen, A.C. and Elven, R., 2004. Polyploidy in Artic plants. Biological Journal of the Linnean Society. 82, pp. 521-536.
Comai, L., 2005. The advantages and disadvantages of being polyploid. Nature Review. 6, pp. 836-846.
Lukens, L.N., Quijada, P.A., Udall, J., Pires, C., Schranz, M.E., Osborn, T.C., 2004. Genome redundancy and plasticity within ancient and recent Brassica crop species. Biological Journal of the Linnean Society. 82, pp. 665-674.
Ohno, S., 1970. Evolution by Gene Duplication. Berlin: Springer-Verlag.
Schwarzbach, E., Donovan, L., Rieseberg, L., 2001. Transgressive character expression in a hybrid sunflower species. American Journal of Botany. 88(2), pp. 270-277.